In the paper industry polyaminamide-epichlorohydrin (PAE) resins are used for developing wet strength in paper products. These resins are described in U.S. Pat. Nos. 2,926,154 and 5,644,021. In general, PAE resins are shipped to the paper mills at a 12 weight percent solution in water. The use of these resins is problematic since, as formulated in the prior art, unacceptable amounts of organic chloride impurities are produced which contribute to the overall environmental problems of certain paper mills. To overcome these problems, extensive research is being conducted to reduce the organic chlorine impurities in the papermaking process and to improve the efficiency of the papermaking operations. Prior art PAE resins have to be shipped in dilute form to the mills and this increases costs to the mill since, in effect, the mill is paying shipping costs for transporting water.
The art has been trying to increase the solids content of an aqueous PAE resin mixture that is shipped to the mill so that freight is not paid on water and inactive resin. Prior art aqueous PAE formulations having a solids content of 20 to 29 weight percent are known but are not preferred for commercial operations since these resins contain a very high amount of organic chloride impurities detrimental to the environment. PAE resins having a solids content in the range of about 40 weight percent in an aqueous formulation have been prepared but these resins are not fully functional. Fully functional in the PAE resin field is defined as a PAE resin having a sufficiently high molecular weight and azetidinium content to provide wet strength equal to or greater than that achieved by prior art PAE at resin concentrations of less than 35 weight percent (usually 12 weight percent). As a practical matter, a fully functional PAE resin of this invention is a resin controlled to exhibit a PAE concentration in excess of 35 weight percent; a Z average molecular weight in excess of 300,000 grams per mole and an azetidinium moiety content in excess of about 1.4 milliequivalents per gram of solids. Usually the resin concentration of this invention is between 35 and 45 weight percent, but upon dilution and other process modifications, the resin concentration can suitably vary from 12 to 45 weight percent advantageously from 12 to 35 weight percent. This resin also comprises less than seven weight percent of residual, non polymeric, chlorinated, organic compounds based on the total solids content of the wet strength formulation and does not jell for four weeks after formulation. This type of a PAE resin is unknown to the prior art.
Our novel thermosettable wet strength resin comprises the product of reaction of an epihalohydrin and an end-capped polyaminamide polymer. The advantageous properties of the PAE resins of this invention are achieved because the epihalohydrin is reacted with an end capped polyaminamide polymer. The end capped polyaminamide polymer in conjunction with epihalohydrin provides a thermosetting resin which has a shelf life in excess of four weeks at a temperature of 25 to 38.degree. C. and has a solids content of 12 to 45 weight percent. The shelf life is much greater at temperatures below 25.degree. C. The shelf life is also greater at solids content of less than 35 weight percent but for optimum commercial use the solids content should be in the range of 35 to 45 weight percent. We define a thermosetting resin as a resin that cross-links through reactive groups inherent to the resin to form structures that cannot be melted or dissolved in water.
Prior art PAE resin formulations are not suitable for modern applications even though the PAE solids content is about 20-29 weight percent since these resins have very high levels of residual (non-polymer) chlorinated organic moieties. This is also true for the PAE resin formulation disclosed in U.S. Pat. No. 5,644,021, which is the most recent prior art reference. Therefore these products are generally environmentally unacceptable in the paper manufacturing process. For a resin to be fully functional as defined above, the resin also has to have a reasonable shelf life. U.S. Pat. No. 5,249,142 discloses a low chlorine PAE product which has a short shelf life and lower wet strength efficiency and is not prepared from a chain terminated polyaminamide polymer. Shelf life is the time span between the manufacture of the PAE resin and the point the resin jells. At a minimum this should be at least three weeks, preferably four or more weeks. The prior art does not disclose or suggest suitable methods for the preparation of a PAE type resin at a solids content in excess of 35 weight percent that is fully functional (as described herein) and has a shelf life of about four weeks.
The present invention provides novel end-capped polyaminamide polymers which when reacted with epichlorohydrin produce PAE resins suitable for use in aqueous thermosettable wet strength resin formulations exhibiting a shelf life of about four weeks at a solids content in excess of 35 weight percent at a temperature of about 25 to 38.degree. C. These resins have a Z molecular weight of about 300,000 grams per mole to 700,000 grams per mole and have an azetidinium moiety content of at least 1.4 milliequivalents. Preferably the azetidinium moiety content is in the range 1.5-1.7 and higher milliequivalents per gram of solid in the thermosettable wet strength resin formulation.
The process for the manufacture of the PAE to produce the high solids content, long shelf life, thermosettable wet strength resin formulations of this invention advantageously produce low levels of non polymeric, chlorinated, organic compounds in the PAE resin formulations, usually less than seven weight percent, preferably less than six weight percent of the total solids. High solids PAE resins having such a low percentage of non-polymeric, chlorinated, organic compounds are not produced by prior art processes.